1. Field of the Invention
The invention relates to an asynchronous transfer mode-passive optical network (ATM-PON) system, more particularly to a device for transmitting and receiving optical signals used in an ATM-PON system, and most particularly to removal of a coherent light in such a device.
2. Description of the Related Art
FIG. 1 illustrates an ATM-PON system. In the ATM-PON system, optical signals are interactively transmitted between a station 201 and subscribers 202 to 206. The station 201 is connected to each of the subscribers 202 to 206 through one-core optical fibers 400.
Herein, optical signals successively transmitted from the station 201 to the subscribers 202 to 206 are called down signals, and burst signals transmitted from each of the subscribers 202 to 206 to the station 201 is called up signals. The up and down signals are designed to have different wavelengths from each other.
Devices 301 to 306 equipped in the station 201 and each of the subscribers 202 to 206 are designed to be able to not only transmit an optical signal, but also receive an optical signal.
Each of the devices 301 to 306 is designed to include a laser diode (LD) from which an optical signal is transmitted.
FIG. 2 is a circuit diagram of a circuit included in each of the devices 301 to 306 for receiving optical signals. The illustrated circuit is comprised of a photodiode 101, a trans-impedance amplifier 102 electrically connected in series to the photodiode 101, and a feedback resistor 103 electrically connected in parallel to the trans-impedance amplifier 102.
The photodiode 101 converts a received optical signal into an electric signal. The trans-impedance amplifier 102 amplifies and converts the electric signal transmitted from the photodiode 101, into a voltage signal. The feedback resistor 103 defines a gain of the trans-impedance amplifier 102.
The above-mentioned devices 301 to 306 are accompanied with a problem that when the laser diode transmits an optical signal, the optical signal to be transmitted strays in the devices 301 to 306, and enters the circuit illustrated in FIG. 2 as a coherent light, resulting in deterioration in photosensitivity in the devices 301 to 306.
When the devices 301 to 306 are of an optical waveguide, in particular, the above-mentioned coherent light exerts much harmful influence on the circuit illustrated in FIG. 2, resulting in more remarkable deterioration in photosensitivity in the devices 301 to 306.
In order to minimize deterioration in photosensitivity in the devices 301 to 306 in their operation, it is necessary to remove a coherent light component from a received optical signal.
In order to remove a coherent light component from a received optical signal, optical countermeasure has been conventionally applied to an optical waveguide. For instance, Japanese Unexamined Patent Publication No. 10-54917 has suggested the formation of a slit in an optical waveguide for interrupting a coherent light from reaching the optical waveguide.
However, it was quite difficult or almost impossible to sufficiently remove a coherent light only by means of optical countermeasures.
Japanese Unexamined Patent Publication No. 5-289120 has suggested an optical waveguide device including a device for combining optical signals to one another and separating optical signals into respective optical signals, an optical directional coupler, and an optical circuit substrate on which the device and the optical directional coupler are formed in monolithic.
Japanese Patent No. 2923884 (Japanese Unexamined Patent Publication No. 10-311875) has suggested a device including a semiconductor laser emitting a laser beam, a laser receiver receiving the laser beam emitted from the semiconductor laser, a filter positioned between the laser receiver and the semiconductor laser and allowing the laser beam to pass therethrough, the filter removing background noises, an angle adjuster adjusting an inclination angle of the filter to change an incident angle of the laser beam into the filter, and a controller controlling the angle adjuster such that the laser beam passes through the filter at a maximum, in accordance with an output signal transmitted from the laser receiver. The controller includes an oscillator, a multiplier multiplying an output of the oscillator by an output of the laser receiver, a differential amplifier amplifying a difference between an output of the multiplier and a reference voltage, and a manipulator overlapping an output of the differential amplifier with an output of the oscillator, and providing a command to the angle adjuster.
Japanese Unexamined Patent Publication No. 11-27215 has suggested an optical communication module to be used in a parent station radially connected to a plurality of child stations through optical devices and making interactive communication with each of the child stations in time division multiplex by switching a mode between a receipt mode and a transmission mode. The parent station includes first output means for transmitting a first reset signal in accordance with a difference in a signal receiving level in the receipt mode, second output means for transmitting a second reset signal when the transmission mode is switched to the receipt mode, and ATC means for automatically setting a threshold level when the first or second reset signal is received.
In view of the above-mentioned problem in the prior art, it is an object of the present invention to provide a device for interactively transmitting optical signals and receiving optical, which is capable of removing a coherent light by which photosensitivity in the device would be deteriorated.
In one aspect of the present invention, there is provided a device for interactively transmitting optical signals in a first wavelength and receiving optical signals in a second wavelength both through a one-core optical fiber, including (a) a first light-electricity signal converter which converts a received optical signal into a first electric signal, (b) a second light-electricity signal converter which converts an optical signal to be transmitted, into a second electric signal, and (c) a coherent light remover which subtracts a level of the second electric signal from a level of the first electric signal to remove a coherent light included in the received optical signal.
There is further provided a device for interactively transmitting optical signals in a first wavelength and receiving optical signals in a second wavelength both through a one-core optical fiber, including (a) a first light-electricity signal converter which converts a received optical signal into a first electric signal, (b) a second light-electricity signal converter which converts an optical signal to be transmitted, into a second electric signal, (c) a delay line which adjusts a phase difference between the first and second electric signals such that the first and second electric signals are in phase with each other, and (d) a coherent light remover which subtracts a level of the second electric signal from a level of the first electric signal to remove a coherent light included in the received optical signal.
For instance, the delay line is positioned between the first light-electricity signal converter and the coherent light remover when the first electric signal reaches the coherent light remover earlier than the second electric signal.
As an alternative, the delay line is positioned between the second light-electricity signal converter and the coherent light remover when the second electric signal reaches the coherent light remover earlier than the first electric signal.
It is preferable that the device further includes a first variable gain amplifier electrically connected in series between the first light-electricity signal converter and the coherent light remover.
For instance, the first variable gain amplifier is comprised of a first impedance amplifier electrically connected in series to the first light-electricity signal converter, and a first variable resistor electrically connected in parallel to the first impedance amplifier.
It is preferable that the device further includes a second variable gain amplifier electrically connected in series between the second light-electricity signal converter and the coherent light remover, the second variable gain amplifier equalizing a level of an output signal transmitted from the second light-electricity signal converter, to a signal level of a coherent light included in the received optical signal.
For instance, the second variable gain amplifier is comprised of a second impedance amplifier electrically connected in series to the second light-electricity signal converter, and a second variable resistor electrically connected in parallel to the second impedance amplifier.
It is preferable that the device further includes a third variable gain amplifier electrically connected in series to the second light-electricity signal converter, and in parallel with the second variable gain amplifier.
For instance, the third variable gain amplifier is comprised of a third impedance amplifier electrically connected in series to the second light-electricity signal converter, and a third variable resistor electrically connected in parallel to the third impedance amplifier.
For instance, the coherent light remover may be comprised of a differential amplifier.
For instance, the above-mentioned device is of a waveguide type.
It is preferable that the device further includes (d) an optical waveguide connected to the one-core optical fiber, (e) a filter for selecting a wavelength, the received optical signal being transmitted through the filter and being received at the first light-electricity signal converter, and (f) a laser diode emitting a laser beam which is coupled to the optical waveguide, and then, output through the optical fiber, the second light-electricity signal converter being located at the rear of the laser diode and receiving a backlight from the second light-electricity signal converter.
There is still further provided a device for interactively transmitting optical signals in a first wavelength and receiving optical signals in a second wavelength both through a one-core optical fiber, including (a) a first photodiode which converts a received optical signal into a first electric signal, (b) a second photodiode which converts an optical signal to be transmitted, into a second electric signal, and (c) a differential amplifier which subtracts a level of the second electric signal from a level of the first electric signal to remove a coherent light included in the received optical signal.
There is yet further provided a device for interactively transmitting optical signals in a first wavelength and receiving optical signals in a second wavelength both through a one-core optical fiber, including (a) a first photodiode which converts a received optical signal into a first electric signal, (b) a second photodiode which converts an optical signal to be transmitted, into a second electric signal, (c) a delay line which delays the first electric signal such that the first and second electric signals are in phase with each other, and (d) a differential amplifier which subtracts a level of the second electric signal from a level of the first electric signal to remove a coherent light included in the received optical signal.
For instance, the delay line is positioned between the first photodiode and the differential amplifier when the first electric signal reaches the differential amplifier earlier than the second electric signal.
As an alternative, the delay line is positioned between the second photodiode and the differential amplifier when the second electric signal reaches the differential amplifier earlier than the first electric signal.
It is preferable that the device further includes a first trans-impedance amplifier which amplifies the first electric signal and converts the first electric signal into a first voltage signal, and a second trans-impedance amplifier which amplifies the second electric signal and converts the second electric signal into a second voltage signal.
It is preferable that the device further includes a first variable feedback resistor electrically connected in parallel to the first trans-impedance amplifier, and a second variable feedback resistor electrically connected in parallel to the second trans-impedance amplifier.
It is preferable that the device further includes a third trans-impedance amplifier electrically connected in series to the second photodiode, and in parallel with the second trans-impedance amplifier, the third trans-impedance amplifier amplifying the first output signal and converting the first output signal into a third voltage signal.
It is preferable that the device further includes a third variable feedback resistor electrically connected in parallel to the first trans-impedance amplifier, and a second variable feedback resistor electrically connected in parallel to the second trans-impedance amplifier.
It is preferable that the device further includes (a) an optical waveguide connected to the one-core optical fiber, (b) a filter for selecting a wavelength, the received optical signal being transmitted through the filter and being received at the first photodiode, and (c) a laser diode emitting a laser beam which is coupled to the optical waveguide and then output through the optical fiber, the second photodiode being located at the rear of the laser diode and receiving a backlight from the second photodiode.
In another aspect of the present invention, there is provided a method of removing a coherent light in a device for interactively transmitting optical signals in a first wavelength and receiving optical signals in a second wavelength both through a one-core optical fiber, including the steps of (a) converting a received optical signal into a first electric signal, (b) converting an optical signal to be transmitted, into a second electric signal, and (c) subtracting a level of the second electric signal from a level of the first electric signal to remove a coherent light included in the received optical signal.
There is further provided a method of removing a coherent light in a device for interactively transmitting optical signals in a first wavelength and receiving optical signals in a second wavelength both through a one-core optical fiber, including the steps of (a) converting a received optical signal into a first electric signal, (b) converting an optical signal to be transmitted, into a second electric signal, (c) adjusting a phase difference between the first and second electric signals such that the first and second electric signals are in phase with each other, and (d) subtracting a level of the second electric signal from a level of the first electric signal to remove a coherent light included in the received optical signal.
For instance, the first electric signal is delayed in the step (c) when the first electric signal is faster than the second electric signal.
As an alternative, the second electric signal is delayed in the step (c) when the second electric signal is faster than the first electric signal.
It is preferable that the method further includes the step of equalizing a level of the first or second electric signal to a signal level of a coherent light included in the received optical signal.
It is preferable that the method further includes the steps of converting the first electric signal into a first voltage signal, and converting the second electric signal into a second voltage signal.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
In accordance with the above-mentioned present invention, only a coherent light included a received optical signal is detected and remove. Accordingly, it is possible to electrically remove a coherent light without applying an optical countermeasure to the device.
In particular, it is quite important for the device including an optical waveguide to prevent deterioration in photosensitivity, caused by a coherent light. Since the present invention removes a coherent light by a simply structured circuit, this is quite effective for the device including an optical waveguide.